In a message posted just after 8 a.m. ET, Trump wrote Sen. Kirsten Gillibrand, D-New York, was a "lightweight" who would "do anything" for campaign contributions. Trump didn't explain further what he meant.

Lightweight Senator Kirsten Gillibrand, a total flunky for Chuck Schumer and someone who would come to my office "begging" for campaign contributions not so long ago (and would do anything for them), is now in the ring fighting against Trump. Very disloyal to Bill & Crooked-USED!

"Lightweight Senator Kirsten Gillibrand, a total flunky for Chuck Schumer and someone who would come to my office "begging" for campaign contributions not so long ago (and would do anything for them), is now in the ring fighting against Trump. Very disloyal to Bill & Crooked-USED!," Trump tweeted.

Trump didn't respond to shouted questions about his message during a midday signing ceremony for a defense spending bill in the White House Roosevelt Room. But later, his spokeswoman insisted that his comments reflected a broader indictment of a corrupt political culture.

"This is the same sentiment the President has expressed many times before," White House press secretary Sarah Sanders said during the daily briefing. "The system is clearly broken and it's clearly rigged for special interests. This President can't be bought and it's one of the reasons he's President today."

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Sanders said Trump had long expressed similar sentiments, including about men, though didn't cite any specific examples.

"He's used that same terminology many times," she said. "There is no way this is sexist at all."

Gillibrand, who an aide said was in a morning Bible study session when Trump posted his message, responded shortly after, tweeting: "You cannot silence me or the millions of women who have gotten off the sidelines to speak out about the unfitness and shame you have brought to the Oval Office."

You cannot silence me or the millions of women who have gotten off the sidelines to speak out about the unfitness and shame you have brought to the Oval Office. https://t.co/UbQZqubXZv

Speaking at a news conference later in the day, Gillibrand called Trump's online insult derogatory.

"I see it as a sexist smear. I mean that's what it is," she said on Capitol Hill. "It's part of the President's efforts of name calling and it's not going to silence me, it's not going to silence me. It's intended to silence me. It's not going to silence the women who have stood up against him directly, and it's not going to silence the millions of women who been speaking out every day since his inauguration about things they disagree with."

Denying any sexual innuendo in Trump's morning tweet, Sanders says any suggestion of impropriety was in the eye of the reader.

"Only if your mind is in the gutter would you have read it that way," Sanders said.

According to Federal Election Commission documents, Trump donated $4,800 to Gillibrand for Senate in 2010 and $2,100 to Gillibrand Victory Fund in 2007. A source close to Gillibrand said the senator did visit Trump in his office in 2010, and noted that his daughter, Ivanka Trump, was there the entire time.

"Are you really trying to bully, intimidate and slut-shame @SenGillibrand?" she wrote. "Do you know who you're picking a fight with? Good luck with that, @realDonaldTrump. Nevertheless, #shepersisted."

Calls for resignation

Trump's online broadside came after Gillibrand told CNN's Christiane Amanpour that Trump should step down as multiple women revive their allegations of sexual assault and harassment after originally launching them during last year's presidential campaign.

"President Trump should resign," Gillibrand said. "These allegations are credible, they are numerous. I've heard these women's testimony, and many of them are heartbreaking."

Trump dismissed those accusations on Tuesday, alleging the claims against him were an invention by Democrats who are unable to prove his campaign colluded with Russia to meddle in the 2016 election.

The morning tweet amounted to Trump's first direct dismissal as President of the sexual allegations that were renewed this week.

Despite thousands of hours wasted and many millions of dollars spent, the Democrats have been unable to show any collusion with Russia - so now they are moving on to the false accusations and fabricated stories of women who I don't know and/or have never met. FAKE NEWS!

"Despite thousands of hours wasted and many millions of dollars spent, the Democrats have been unable to show any collusion with Russia - so now they are moving on to the false accusations and fabricated stories of women who I don't know and/or have never met. FAKE NEWS!" Trump wrote on Twitter at 7:10 a.m. ET.

Trump's argument that he'd never met his accusers doesn't hold up to scrutiny; one of the women interviewed him for a magazine article and another was a contestant on the television show he hosted.

The message combined two topics that have dogged Trump over the course of his first year in office. While the Russia investigation has clouded his administration for months, the charges of sexual misconduct have recently gained new prominence as a wave of similar allegations have led to resignations and firings of powerful men in Congress, Hollywood and the media.

On Monday, three women who accused Trump of sexual harassment or sexual assault detailed their stories anew, claiming Trump visually inspected, groped, fondled or forcibly kissed them during his career as a reality television star and real estate developer.

White House dismisses allegations

The women are among the at least 15 who came forward with a wide range of accusations against Trump. The White House rebutted the women on Monday, saying Trump himself had denied the allegations.

"This took place long before he was elected to be president," White House press secretary Sarah Sanders said. "And the people of this country, at a decisive election, supported President Trump, and we feel like these allegations have been answered through that process."

That message ran counter to at least one other member of Trump's administration -- Nikki Haley, the US ambassador to the United Nations -- who said on Sunday that Trump's accusers deserve to be heard.

"I know that he was elected," Haley told CBS. "But, you know, women should always feel comfortable coming forward. And we should all be willing to listen to them."

Trump feels "victimized" by the sexual misconduct allegations, his friend Chris Ruddy said on CNN Tuesday.

"I think Donald Trump looks at this and says I was victimized by this," said Ruddy, the CEO of Newsmax who speaks often with the President. "I was in public life for 30 years...and yet there was never a single allegation, both public and private, against him, in 30 years."